1. Field of the Invention
The present invention relates to a mold for injection molding which is used at the time of injection molding the disc substrate of an optical disc.
2. Related Art
An optical disc as the representative of a CD (compact disc) etc. is configured in a manner that a reflection film and a protection film are laminated on a disc-shaped transparent substrate (hereinafter such a transparent substrate is called as a disk substrate) having a recording surface on which a pit sequence with recording information such as music information, image information etc. is transferred. The recording information recorded on the optical disc thus configured is optically read by irradiating an optical beam on the recording surface from the disc substrate side and receiving a reflection beam reflected from the pit sequence on the recording surface.
Thus, the disc substrate is required to be uniform in its optical characteristics such as an optical transmittance, a refractive index etc. To this end, amorphous plastic material such as PC (polycarbonate), PMMA (polymethyl methacrylate) which is transparent and easy to obtain a uniform refractive index is employed as molding material. Such molding material is subjected to the molding process by the injection molding apparatus using the mold for injection molding to thereby form a disc substrate.
FIG. 4 is a sectional view showing the schematic diagram of a mold K0 of the injection molding apparatus which molds the disc substrate of a CD. As shown in this figure, such mold K0 of the conventional apparatus includes a pair of a fixed mold body 101 and a movable mold body 102.
The fixed mold body 101 and the movable mold body 102 are subjected to the mold clamping in a manner that the mold forming surfaces thereof are opposed to each other to thereby define a mold space 105 of the substrate to be molded.
The fixed mold body 101 includes a fixed mold mirror surface board 106 of a disc shape which forms one surface of the disc substrate to be molded, and is fixed by means of a screw etc. to a not-shown fixed side die plate of the main body of the injection molding apparatus through an attachment mold board 107. A tubular spool bush 108 is passed through the fixed mold mirror surface board 106 and inserted in the center portion within the fixed mold mirror surface board 106. The spool bush 108 is configured as a mold which is exchangeable and provided separately from the fixed mold mirror surface board 106. The spool bush is formed by metal material of generally used SUS system like the fixed mold body 101 and the body 02. A spool 109 for conducting the molding material to the mold space 105 is provided within the spool bush 108. The fixed mold body 101 is fixed by a not-shown holding member so that the injection hole of the nozzle of a not-shown injection mechanism for injecting the molding material coincides with the conduction hole of the spool 109. The spool 109 forms a conduction path for conducting molten molding material injected from the nozzle within the mold space 105.
The movable mold body 102 is provided with a board of a disc shape having the other surface of the disc substrate to be molded, that is, the surface to which a stamper 110 for forming pits carrying recording information is attached. The movable mold body is fixed by means of screws etc. to a not-shown movable side die plate through an attachment mold board 112. A tubular inner peripheral clamp 113 constituting a part of the movable mold body 102 and an annular-shaped outer peripheral clamp 114 are provided at the portions corresponding to the inner periphery and the outer peripheral edge portion of the stamper 110 on the mold forming surface of the movable mold mirror surface board 111, respectively. The inner peripheral clamp 113 has an annular projection portion of an almost rectangular shape in section which protrudes within the mold space 105. The projection portion presses the inner peripheral portion of the stamper 110 against the movable mold mirror surface board 111 and fixes thereto. The outer peripheral clamp 114 has a mold forming surface for defining the outer peripheral edge portion of the substrate to be molded. The stamper 110 is attached to the movable mold mirror surface board 111 by the inner peripheral clamp 113 and the outer peripheral clamp 114 to thereby form a part of the mold forming surface of the movable mold mirror surface board 111.
As shown in FIG. 4, coolant grooves are provided within each of the movable mold mirror surface board 111 and the fixed mold mirror surface board 106. A tubular punch 115 is provided at the center portion of the inner periphery of the movable mold mirror surface board 111 so as to be surrounded by the inner peripheral clamp 113 and freely movable in the direction shown by an arrow S in the figure. The tip portion of the punch 115 forms a part of the mold forming surface. That is, in this case, the tip portion of the punch forms a portion corresponding to the center hole of the substrate which is formed by cooling and solidifying the molten molding material that is conducted and filled within the mold space 105.
The punch 115 is coupled to the output shaft of an oil pressure cylinder mechanism (not shown), whereby the punch is driven in the direction shown by the arrow S. Thus, the punch 115 is driven in the direction shown by the arrow S after the substrate is molded within the mold space 105, so that the tip end portion of the punch can perforate the substrate to thereby punch the portion corresponding to the center hole of the substrate.
An eject pin 116, for separating from the punch 115 the portion corresponding to the center hole of the substrate punched by the punch 115, is provided within the punch 115 so as to reciprocally move freely in the direction shown by the arrow S. The eject pin 116 is also driven by the not-shown oil pressure cylinder mechanism. These mechanisms form a cutting mechanism constituting a part of the movable mold body 102.
An annular ejector 117 constituting a part of the movable mold body 102 is provided between the inner peripheral clamp 113 and the punch 115. The ejector 117 serves to push the substrate thus molded and exfoliate from the movable mold mirror surface board 111. The ejector is freely movable in the direction shown by the arrow S and also driven by the not-shown oil pressure cylinder mechanism.
On the other hand, at the tip end portion of the spool bush 108 provided so as to penetrate the center of the fixed mold mirror surface board 106 of the fixed mold body 101, a die portion 119 constituting a part of the mold forming portion is formed so as to oppose to the punch 115 of a mold releasing mechanism. The portion 119 serves to punch the center hole of the substrate in cooperation with the tip end portion of the punch 115. A space formed among the die portion 119 of the spool bush 118, the tip end portion of the spool bush inside the die portion 119 and the tip end portion of the punch 115 constitutes a runner and a gate serving as a fluid path for conducting the molten molding material injected from the spool 109 into the mold space 105.
Next, the operation of the injection molding apparatus using the mold K0 will be explained.
In the injection molding apparatus, at first, in order to make the temperature distribution uniform within the mold space 105 formed by putting the fixed mold body 101 and the movable mold body 102 together to thereby smoothly flow the molten molding material therein, coolant material such as water, oil etc. which is adjusted so as to have a predetermined temperature equal to or lower than the heat deformation temperature of the molten molding material injected into the mold space 105 is circulated within the coolant grooves of the fixed mold body and the movable mold body so as to preliminarily heat the mold space.
When the mold K0 is kept at the predetermined temperature, then, the fixed mold body 101 and the movable mold body 102 are subjected to the mold clamping. Thereafter, as the injection process, the pressurized molten molding material is injected into the spool 109 from the nozzle of the injection mechanism coupled to the spool 109, then injected within the mold space 105 held at the predetermined temperature in a short time and filled therein. Thus, the molding material is cooled and solidified at a temperature less than the heat deformation temperature in a state where the mold K0 is kept in the mold clamping state.
In this case, since the temperature within the mold space 105 is kept at the predetermined temperature, the molten molding material just after being filled within the mold space 105 shrinks in its high-polymer chain and so solidifies at the temperature equal to or lower than the heat deformation temperature. As a result, since the mold K0 is cooled forcedly in a short time, the molten molding material filled within the mold space 105 is cooled rapidly and so solidified in a disc shape in a state that the pit shapes of the stamper 110 are transferred to the molding material.
In this case, since the molten molding material at the portion corresponding to the inner peripheral side portion from the center hole of the substrate having been filled in the fluid path such as the runner and the gate communicating with the mold space 105 is also cooled and solidified simultaneously, the inner peripheral side portion from the center hole of the substrate is formed in a state being coupled to the disc shape portion. The injection molding apparatus pulls out the center hole portion of the solidified substrate by the operation of the cutting mechanism to form the center hole of the disc substrate.
In this manner, the disc substrate is formed.
In this manner, according to the injection molding apparatus for a disc substrate, at each molding cycle of the substrate, the disc substrate is formed in a manner that the pressurized molten molding material is injected and filled in a short time within the mold space 105 of the preliminarily heated mold K0 to thereby solidify the molten molding material. In this respect, each of the spool bush and the punch is set to a temperature lower than that of the fixed type mold mirror surface board by a not-shown cooling mechanism.
(Problems that the Invention is to Solve)
However, as described above, in the mold K0, since the spool bush 108 for conducting the molten molding material within the mold space 105 is inserted so as to penetrate the center of the fixed mold mirror surface board 106, and at the time of the molding injection of the substrate, heat radiated from the molten molding material sequentially injected into the spool 109 from the nozzle of the injection mechanism in the injection process is transmitted to the spool bush 108 and thereafter sequentially transmitted to the fixed mold mirror surface board 106 adjacent to the spool bush 108. As a result, the portion adjacent to the spool bush 108 of the fixed mold mirror surface board 106 which forms the one surface of the substrate becomes higher in temperature than other portion of the fixed mold mirror surface board 106 and the movable mold mirror surface board 111.
Thus, the portion adjacent to the spool bush 108 of the fixed mold mirror surface board 106 becomes lower in the decreasing rate of the temperature as compared with the other portion of the fixed mold mirror surface board 106 and the movable mold mirror surface board 111. In other words, the temperature distribution within the mold does not become uniform.
As a result, in the substrate, since the portion near the center hole of the substrate on the mold forming surface side formed by the movable mold mirror surface board 111 is molded in a state that the temperature of this portion is kept higher than that of other molding surface including the molding surface on the opposite side, this portion is molded until the high-polymer chain thereof is sufficiently shrunk. That is, the substrate is molded in the same configuration as the mold space 105 while remaining much residual stress near the center hole of the substrate on the mold forming surface side molded by the movable mold mirror surface board 111.
In this manner, since the residual stress becomes asymmetrical between the fixed mold mirror surface board 106 side and the movable mold mirror surface board 111 side, in the disc substrate thus molded, the surface on the fixed mold mirror surface board 106 side on which much residual stress distortion is held just after the molding shrinks gradually with much time longer than the opposite side, that is, the surface on the movable mold mirror surface board 111 side. Thus, there may arises a large warp in the disc radial direction.
When a pickup reads, from the inner periphery side to the outer periphery side by means of a laser beam, the recorded information on the information recording surface of an optical disc which is formed by a disc substrate with such a large warp, the objective lens of the pickup inclines relatively with respect to the disc substrate in accordance with the degree of the warp. Thus, the optical axis of the laser beam incident on the disc substrate inclines from the direction perpendicular to the disc substrate. This inclined angle from the direction perpendicular to the disc substrate (tilt angle) becomes larger in accordance with the degree of the warp at the each radial position of the disc substrate. When the tilt angle of the incident beam becomes larger, there arises in the reflection beam coma aberration having the magnitude according to the tilt angle. Thus, there arises a problem that it is difficult to accurately read recorded information at the largely warped position of the disc substrate of the optical disc.
The coma aberration becomes larger depending on the magnitude of the NA (numerical aperture) of the objective lens of the pickup and the wavelength of the laser beam. Thus, when reading a reflection beam from pits formed on the disc substrate of a DVD etc. on which information is recorded with a higher density, the tilt margin of the pickup, that is, the allowable range of the tilt angle of the incident beam of the pickup with respect to the disc substrate becomes smaller. As a result, pickups used for high-density recording discs such as DVDs are required to adjust the radial skew in a restricted range as compared with pickups for CDs.